Here's a trivially easy way to create your own VOR Test transmitter.
What you need:
A Raspberry Pi (any model will do, including the $5 Pi Zero)
A laptop, or cellphone (I presume you already have this)
A piece of wire, about 18" long, length not critical
A 3.5mm jack plug
On the Pi, download and install rpitx: https://github.com/F5OEO/rpitx
If you're using a model A pi, or a pi zero you may need to engage this hack:
https://github.com/F5OEO/rpitx/issues/5
EDIT: for clarity - this is not my software.
rpitx turns the pi into a somewhat versatile SDR transmitter. We'll use it to generate a steady 108.00MHz carrier. Terrible harmonics in the output, but it's free, so don't complain.
Download the attached audio file:
Attachment:
http://www.avcanada.ca/forums2/download ... p?id=11302
Downloaded 12 times
That file is 10 seconds of the synthesized baseband signal of what a VOR transmits on the 0 degree radial. (Only 10 seconds because this brain-dead forum software won't take a file larger than 1 MByte. So when it comes time to play back the file, loop it, ok?). There's also an on-off-on-off 1020hz ident tone, so you can tell you're tuned to the right frequency.
Execute the following command on the pi:
sudo ./rpitx -m VFO -f 108000
That gives you a 108.00MHz carrier from pin 12 (http://pi4j.com/pins/model-a-plus.html). All we need to do now is modulate the audio onto the carrier and fire it into a bit of wire.
As it turns out the easiest way to get some modulation going is to connect pin 12 on the pi to the tip (or ring) of the headphone output of your (my) laptop or iPhone, and wind up the volume. Any kind of non-linear mixing will cause modulation, and as good luck will have it, at about 3/4 full volume there's enough non-linear stuff going on either in the pi or the audio output stage of the laptop to get a decent depth of modulation. I used a wire about 18 inches long to connect the two, which also acts as the transmission antenna.
Since I was powering the pi from a USB port on the laptop, I didn't need to connect the sleeve (GND) of the headphone output as well. If the pi is running from another power source, you should probably connect the GND of the headphone output to a GND pin on the pi. There are lots to choose from.
Caveat: yes, you're firing a 108MHz 5v square wave up the jacksie of your laptop. Doesn't seem to have troubled mine very much, nor my iPhone. But this is at your own risk.
Stretch the wire out straight, and play the audio. That's it.
Turn on and tune in the VOR receiver to 108.0 MHz. The needle should centre with a 'from' flag on the 0 radial, and a 'to' flag on the 180 radial. (One Eight Two = one eight, to, remember?)
This morning I had four aircraft in the hangar all happily tuned in and confirming to me that their VOR resolvers were nicely lined up, with the setup as described. Plenty of range. Clear audio idents. I have synthesized files for the 45 and 90 degree radials too, and any others you want. Adjust the volume on the laptop or iPhone to get decent modulation and a clear ident, as necessary. (If you want the python code to generate the baseband signals yourself, let me know and I'll post it.)
This general idea also works well for checking the localizer receiver, and should work on the glideslope as well, with appropriate audio files and an appropriate carrier frequency. I can explain how to get the audio files right, in another post, if anyone's interested.
The obvious thing to do next is to get rid of the laptop and find another way to do the audio and modulation. Unfortunately, the AM mode on rpitx isn't clean enough by itself. I'll work on this next.
Finally: do NOT go all Colonel Stuart (Die Hard 2) on me and use this setup to interfere with a real VOR, localizer or glideslope frequency in use at an airport. That would not be even slightly funny. Even so, this is all probably illegal - don't tell Industry Canada you're transmitting (even low power) in the aviation bands from an unlicensed transmitter. Naughty naughty.
http://www.avcanada.ca/forums2/viewtopi ... 7&t=107650
